1. Field of the Invention
The present invention relates to a projection type exposure apparatus which is used to transfer a mask pattern onto a photosensitive substrate in a photolithographic process used to manufacture semiconductor devices, imaging devices (CCD's, etc.), liquid crystal display devices, and thin-film magnetic heads, etc. More particularly, the present invention relates to the a scanning exposure apparatus such as a projection type exposure apparatus using a step-and-scan exposure scheme in which a mask pattern is successively transferred onto respective shot areas on a photosensitive substrate by the synchronized scanning of the mask and the photosensitive substrate.
2. Discussion of the Related Art
Conventionally, in manufacture of semiconductor devices, etc., a reduction projection type exposure apparatus (steppers) using a step-and-repeat exposure scheme (one-time exposure system) has been widely used as a projection type exposure apparatus to transfer a reticle pattern (mask pattern) on a mask onto respective shot areas of a wafer coated with a photoresist. Recently, a projection type exposure apparatus using a so-called "step-and-scan" exposure scheme has attract considerable attention. In the step-and-scan exposure apparatus (or scanning exposure apparatus), reduced images of a pattern on a reticle are successively transferred onto respective shot areas of a wafer and are exposed by scanning the reticle and wafer in synchronization with each other with respect to a projection optical system. The step-and-scan exposure apparatus projects a portion of the pattern on the reticle onto the wafer via the projection optical system to meet the recent requirement that larger pattern images be transferred to the wafer without using a large or complicated projection optical system. In conventional aligners, in which the pattern on the entire surface of the reticle is transferred onto the entire surface of the wafer as a positive, unit-magnified image through a one-time scanning exposure using an integral type stage system, the structure of the apparatus is rather simple. In the step-and-scan exposure apparatus, however, it is necessary for the reticle stage and the wafer stage to move at different speeds in accordance with the projection magnification. Furthermore, since movement between the shot areas on the wafer surface is performed by stepping motion, the stage system (reticle stage and the wafer stage) becomes very complicated, and also extremely high accuracy is required in controlling the stage system and the exposure apparatus itself.
Specifically, in the case of a projection type exposure apparatus using a step-and-scan scheme, it is necessary that the reticle stage and the wafer stage be scanned independently and in a stable manner with both stages synchronized in a predetermined positional relationship. Conventionally, therefore, the following method has been used, for example. Prior to the initiation of scanning, the reticle stage and the wafer stage are aligned. Then, in synchronization with the movement (scanning) of the wafer stage at a predetermined speed in a predetermined direction, the reticle stage is moved (scanned) at a scanning speed corresponding to the predetermined speed. At the same time, the positional deviations of both stages in the scanning direction and non-scanning direction (i. e., the direction perpendicular to the scanning direction) are determined by calculation, and the position of the reticle stage, for example, is finely adjusted to reduce the positional deviations thus determined.
In the method described above, positional deviations of the reticle stage and wafer stage in the scanning direction and non-scanning direction are determined at the time of scanning exposure, and the exposure apparatus is controlled so that these positional deviations are independently corrected. Accordingly, the correction of positional deviations is performed independently in the scanning direction and the non-scanning direction even in the case where the wafer stage is rotated during scanning exposure by yawing, for example. As a result, a positional deviation (synchronization error) is generated between the reticle and the wafer.
In regard to this problem, it is possible to detect the relative rotational angle between the reticle stage and the wafer stage; and control the system such that this rotational angle is maintained at a predetermined target value. However, in the case that positional deviations in the translational direction and the rotational angle are corrected independently, since positional deviations in the translational direction are also generated by rotation, a long correction time (adjustment time) is required in order to bring such synchronization errors within a permissible range.
When superimposing exposure is performed for respective shot areas on the wafer in a one-time exposure system, the superimposition error can be reduced by correcting the wafer position in accordance with the shot arrangement of the wafer, etc., for example. In the case of a scanning exposure system, however, the reticle and wafer are scanned in synchronization with each other. Accordingly, if the wafer is warped or deformed, there is a possibility that the abovementioned synchronization error will increase if the position of the reticle or wafer is simply subjected to a fine adjustment to correct the superimposition error.